1. Field of the Invention
The present invention relates to a film thickness measuring apparatus for measuring the thickness of a thin film on a sample, such as the surfaces of a semiconductor wafer, reticle/mask and a glass substrate of a liquid crystal display (LCD), and more particularly, to an apparatus to enable the measurement of optical characteristics and film thickness of a relatively small surface area, while minimizing chromatic aberrations.
2. Description of Related Art
Film thickness measuring apparatuses have been utilized, such as a spectroscopic ellipsometer that can measure a polarization state change when light reflects from the surface of a substance. By analyzing the detected reflected light, it is possible to measure certain optical constants, such as refractive index and extinction coefficients of the substance. Additionally, when a thin film layer exists on the surface of the substance or sample, it is possible to measure the film thickness and the optical constants.
As semiconductor elements and optical members have become increasingly small, problems have occurred in attempting to measure a film thickness of such a sample, for example, a semiconductor wafer that has a very restricted area. As the field of view or target area of the sample is reduced in size, the diameter of the incident light beam has been frequently reduced correspondingly. An incident optical system used in such a spectroscopic ellipsometer has included a slit provided between a light source and a polarizer, with the size of the aperture of the slit being selected to reduce the diameter of the incident beam upon the sample to be as small as possible.
However, when a light of multiple wavelengths is used as the irradiating light, chromatic aberrations can arise when using a lens for a beam reducing optical system, or using a polarizer as a polarizing element. Thus, it becomes difficult to reduce the incident beam to an extremely small diameter in contrast to the situations where a single wavelength beam is used in other measuring instruments. When the wavelength of the irradiating light ranges from a short wavelength of 190 nm to a long wavelength of 830 nm, it has been found that the reduction of the incident beam diameter can cause considerable difficulty. Incidentally, the foregoing chromatic abberation results from the difference of the refractive index depending upon the wavelengths, i.e., aberrations (defect produced when an image optical system does not satisfy the condition of Gaussian imaging) caused by the dispersion of rays of light.
Even when the incident beam diameter is reduced, there is still a disadvantage in that a secondary light can be developed around the incident light and that this developed light is also added to the measured light in the optical system.
Thus, there is still a desire to improve the ability to measure a film thickness of relatively small restricted areas while avoiding the problems of the prior art.
The present invention provides a film thickness measuring apparatus comprising a source of light with an incident optical system capable of polarizing and focusing the light on a sample location. The incident optical system can comprise a pair of concave reflective surfaces to assist in focusing the light along with a polarizer unit. A member having a slit aperture can also be used to define the incident polarized light beam. A detector optical system can focus the reflected polarizing light for analysis by a spectroscope. The detector optical system can include a phase modulation element and an analyzer element, along with a pair of reflective concave surfaces for focusing the light onto a restricted aperture that defines a relatively small field of view on the sample location. A rotatable member that can be driven, for example, by a stepper motor can selectively position one of a plurality of restricted apertures of respectively different sizes to meet the requirements of the particular sample surface under inspection. A fiber optic can be operatively positioned to align with the selected restricted aperture for providing an input of the focused reflected light to the spectroscope.
Additionally, a microscope and a CCD camera can provide an image of the sample surface for display. A computer unit can process not only the output from the spectroscope to provide the various optical constants, such as a refractive index and extinction coefficient and also the film thickness, but also can further process the image signals from the CCD camera to provide a combined display image to the operator.